Cellulose comprises the major part of all plant biomass. The source of all cellulose is the structural tissue of plants. It occurs in close association with hemicellulose and lignin, which together comprise the major components of plant fiber cells. Cellulose consists of long chains of beta glucosidic residues linked through the 1,4 positions. These linkages cause the cellulose to have a high crystallinity and thus a low accessibility to enzymes or acid catalysts. Hemicellulose is an amorphous hetero-polymer which is easily hydrolyzed. Lignin, an aromatic three-dimensional polymer, is interspersed among the cellulose and hemicellulose within the plant fiber cell.
It has been estimated that about three quarters of the approximately 24 million tons of biomass generated on cultivated lands and grasslands is waste. The utilization of such waste materials for developing alternate sources of fuels, chemicals and other useful products has long been desired. However, attempts to hydrolyze cellulose have not yet succeeded in providing an economically viable method for producing high yields of sugars, due primarily to the crystalline structure of cellulose and the presence of lignin therein.
Previously reported processes for hydrolysing cellulose include biological and non-biological means of depolymerization. The biological methods involve the use a cellulase enzyme. The oldest and best known non-biological method of producing sugars from cellulose is the use of acid hydrolysis. The acid most commonly used in this process is sulfuric acid. In general, sulfuric acid hydrolysis can be categorized as either dilute acid hydrolysis or concentrated acid hydrolysis.
The dilute acid processes generally involve the use of 0.5% to 15% sulfuric acid to hydrolyze the cellulosic material. In addition, temperatures ranging from 90.degree.-600.degree. Celsius, and pressure up to 800 psi are necessary to effect the hydrolysis. At high temperatures, the sugars degrade to form furfural and other undesirable byproducts. The resulting glucose yields are generally low, less than 50%. Accordingly, the dilute acid processes have not been successful in obtaining sugars from cellulosic material in high yields at low cost.
The concentrated acid processes have been somewhat more successful, producing higher yields of sugar. These processes typically involve the use of 60% to 90% sulfuric acid to effect hydrolysis. These processes, although successful at producing sugar yields above 90%, have not been implemented commercially in the past due to the high cost of concentrated sulfuric acid and its subsequent recovery, the difficulties encountered in handling concentrated sulfuric acid, and the need for equipment resistant to the acid at high temperatures. In addition, the higher the acid concentration used, the more energy required to concentrate the acid, resulting in these processes being economically disadvantageous.
More recently, however, the concentrated acid hydrolysis process has become the focus of additional research. (See L. T. Fan, M. M. Gharpuray and Y. H. Lee, Cellulose Hydrolysis, p. 170-172, 1992 and J. D. Broder, J. W. Barrier and G. R. Lightsey, "Conversion of Cotton Trash and Other Residues to Liquid Fuel", presented at the Conference of the American Society of Agricultural Engineers, Dec. 14-15, 1992.) Such processes generally consist of the following stages: (1) prehydrolysis to hydrolyze the hemicellulose portion, (2) main hydrolysis to hydrolyze the cellulose, and (3) post hydrolysis to form glucose from oligosaccharides formed in step (2). The first step involves the addition of sulfuric acid to the biomass which is then heated to at least 100.degree. C. to break down the hemicellulose. The result of this prehydrolysis step is a solution containing not only virtually all of the C.sub.5 sugars, but also C.sub.6 sugars. These C.sub.6 sugars are thus not recovered if the C.sub.5 sugar stream is not utilized, resulting in lower sugar yields. After the sugar stream produced by the prehydrolysis step is removed, concentrated acid is added to disrupt the crystalline lattice of the cellulose and form glucose. The sugars produced are then fermented to alcohols. It has been recognized, however, that in order to commercialize such a process, the steps must be simplified, the energy consumption reduced, and the difficulties encountered in recycling spent acids eliminated.
Additional problems faced in the commercialization of known acid hydrolysis processes include the production of large amounts of gypsum when the spent or used acid is neutralized. The low sugar concentrations resulting from the processes require the need for concentration before fermentation can proceed. When hydrolysis is carried out at temperatures above 150.degree. C., compounds such as furfural are produced from the degradation of pentoses. These compounds inhibit fermentation, and some are toxic.
In addition to these difficulties, it has been recognized that the fermentation of the sugars produced by concentrated acid hydrolysis presents additional problems. The hydrolysis of cellulose and hemicellulose results in the production of both C.sub.5 and C.sub.6 sugars. The hexose sugars are known to ferment easily, while the pentose sugars are generally more difficult to ferment. Thus, the resulting sugars must first be separated, which often involves the use of complicated separation techniques, and then fermented by different microorganisms known to ferment either hexose or pentose sugars alone.
Previous acid hydrolysis processes have not taken into account how biomass containing high amounts of silica are to be treated. Disposal of the silica poses a potential environmental and economic liability. In projects that use biomass to generate energy by combustion, high silica means high slagging tendency, as well as problems with handling large quantities of ash produced when the biomass is burned. Yet silicon compounds are of great commercial importance, and the recovery of silica from agricultural waste has become increasingly important. (See A. Karera, S. Nargis, S. Patel and M. Patel, "Silicon Based Materials from Rice Husk", Journal of Scientific & Industrial Research, Vol. 45, 1986, pp. 441-448.) It is well known that treatment of the biomass with sodium hydroxide will dissolve cellulose and hemicellulose, allowing their separation from the lignin. However, small chain cellulosics often contaminate the silica product during the removal process, thus lowering the sugar yield. In addition, the removal of the silica, done by filtration, is hampered by the formation of a thick gel which is very difficult to filter.
Thus, there is an urgent need for an economically viable, environmentally safe process for producing sugars from biomass containing cellulose and hemicellulose.